Macromolecular delivery into mammalian cells is an attractive approach for cell manipulation, as it would allow modulation of gene expression and modification of the genome, which, in turn, would open new avenues for research and enable the therapeutic targeting of molecules currently viewed as “undruggable” by small molecules. In particular, recombinant nucleases targeting genes or alleles associated with disease have great potential as therapeutic agents. The current methods of macromolecular delivery include viral delivery of nucleic acid molecules, receptor-mediated delivery of nucleic acids or proteins, and the use of protein fusions with cell-penetrating peptides such as TAT, Arg9, or Penetratin for the delivery of proteins. Each of these delivery systems offers benefits for particular applications; in most cases, however, questions regarding efficacy, cytotoxicity, and ease of preparation remain. Easily prepared reagents capable of effectively delivering macromolecules (e.g., functional effector proteins) to a variety of cell lines without significant cytotoxicity or other adverse side effect remain of considerable concern.
Most proteins do not spontaneously enter mammalian cells and are thus naturally limited in their use as research tools and their potential as therapeutic agents. Techniques for the delivery of proteins into mammalian cells have been developed recently to address intracellular targets. These techniques include the use of lipid-based reagents (Zelphati et al., J. Biol. Chem. 276, 35103-35110, 2001), nanoparticles (Hasadsri et al., J. Biol. Chem., 2009), vault ribonucleoprotein particles (Lai et al., ACS Nano 3, 691-699, 2009); genetic or chemical fusion to receptor ligands (Gabel et al., J. Cell Biol. 103, 1817-1827, 1986; Rizk et al., Proc. Natl. Acad. Sci. U.S.A. 106, 11011-11015, 2009); and fusion to cell-penetrating peptides (Wadia et al., Curr. Protein Pept. Sci. 4, 97-104, 2003; Zhou et al., Cell Stem Cell 4, 381-384, 2009). Perhaps the most common method for protein delivery is genetic fusion to protein transduction domains (PTDs) including the HIV-1 transactivator of transcription (Tat) peptide and polyarginine peptides. These cationic PTDs promote association with negatively charged cell-surface structures and subsequent endocytosis of exogenous proteins. Both Tat and polyarginine have been used to deliver a variety of macromolecules into cells both in vitro and in vivo (Wadia et al., Curr. Protein Pept. Sci. 4, 97-104, 2003; Zhou et al., Cell Stem Cell 4, 381-384, 2009; Myou et al., J. Immunol. 169, 2670-2676, 2002; Bae et al., Clin. Exp. Immunol. 157, 128-138, 2009; Schwarze et al., Science 285, 1569-1572, 1999). Despite these advances, intracellular targets remain difficult to affect using exogenous proteins, and even modest success can require toxic concentrations of the respective transduction agent due to the low efficiency with which proteins are functionally delivered into cells (Zhou et al., Cell Stem Cell 4, 381-384, 2009; Wang et al., Nat. Biotechnol. 26, 901-908, 2008). Therefore, there remains a need for better delivery systems for getting functional effector proteins into cells to target intracellular biomolecules.